FIELD OF THE INVENTION
The invention relates to a process for producing a catalyst with a catalytically active composition on a support body by thermal spraying and a catalyst produced according to that process.
U.S. Pat. No. 3,271,326 discloses an essentially nickel-containing catalyst in which a catalytically active surface is applied by flame spraying. In that case, in a first process step, a steel support structure which had previously been mechanically roughened has aluminum flame sprayed on to it for the purpose of further roughening. Subsequently, in a second process step, the catalytically active components are applied by flame spraying to the support structure which was pretreated in that way. A characteristic of that complicated production process is that the catalytically active components which are sprayed on are heated up to their melting point.
Furthermore, German Published, Non-Prosecuted Patent Application DE 38 13 312 A1 discloses applying a titanium dioxide modified with catalytically active components as additives to a metallic structure by thermal spraying. In that procedure, the catalytic ally active components melt briefly and upon cooling fuse together at contact points. That creates a catalytically active surface held together by adhesion forces.
However, it is known that the catalytic activity of the titanium dioxide is very strongly dependent on the proportion of the titanium dioxide which is present in the anatase modification. The anatase modification of titanium dioxide in that case has the property of being converted irreversibly by high temperatures into the less active rutile modification. Nuclei of titanium oxide in the rutile modification in the crystal microstructure grow further upon heating at the expense of the anatase modification.
On that subject, German Published, Non-Prosecuted Patent Application DE 39 16 398 A1 discloses mixing a very low-melting component into the spraying material containing a thermally sensitive component such as titanium dioxide in the anatase modification. The melting point of the very low-melting component in that case is below an inactivation temperature above which, upon influence by a chemical change, for example, the amount of the thermally sensitive component is irreversibly reduced. During thermal spraying, the spraying material is heated only to the melting point of the very low-melting component, so that inactivation of the thermally sensitive component is avoided. The adhesion of the components of the spraying material to one another occurs due to the very low-melting component flowing around other particles and fusing together during the thermal spraying. That enables a surface having a high catalytic activity to be achieved by thermal spraying, even in the presence of a thermally sensitive component.
However, a very low-melting component flowing around a catalytically active component reduces the specific surface area or BET surface area of the composition being applied. In order to achieve a high catalytic activity, the layer thickness of the catalytically active composition being applied accordingly has to be increased. The longer spraying time resulting therefrom can then easily lead to thermal distortion of the support body or the support structure. The process is therefore only suitable for support structure thicknesses of more than about 3 mm.